Adipic acid is commercially produced by the oxidation of cyclohexanol and cyclohexanone by concentrated nitric acid. Initial product recovery and purification are accomplished through crystallization of the reaction mixture followed by solid-liquid separation. Additional purification of adipic acid is accomplished through one or more steps of aqueous recrystallization followed by solid-liquid separation. The reaction of cyclohexanol and cyclohexanone with nitric acid produces adipic acid with several byproducts, e.g. glutaric acid, succinic acid, of varying concentrations. In order to produce adipic acid of good quality, a major part of the purification needs to be accomplished during the initial product recovery through crystallization of the reaction mixture from the nitric acid oxidation step.
For crystallization of adipic acid from the solution obtained from the oxidation of cyclohexanol and cyclohexanone by nitric acid, it has generally been observed that the higher the temperature of crystallization, the purer the product of crystallization. However, the mother liquor from crystallization at high temperature contains a high concentration of adipic acid. In order to prevent loss of adipic acid with the mother liquor of crystallization, most refining processes in commercial practice require that this crystallization be conducted at a low temperature, typically below 60 degrees C. As a result, the product obtained contains substantial impurity that requires significant additional refining. It would, therefore, be beneficial to have a process to achieve the benefit of high temperature crystallization without excessive product loss in the mother liquor.